Integrated circuits have many devices which must be electrically connected to each other. The size of the circuits is presently such that all of the electrical interconnections can not be on the same level. Rather, the electrical interconnections are on different levels which are separated from each other by a layer of a dielectric. Contacts between levels are made by patterning the dielectric to form windows which expose portions of the underlying interconnect material and then filling the window with a conducting material, etc. Of course, contacts are made for other reasons as well such as the contacts made to source/drain regions and the gate of a field effect transistor.
A frequently used material for the electrical interconnect is a metallic silicide such as titanium silicide. This material has a conductivity higher than that of polysilicon and is also frequently used on the source/drain regions and the gate structure. A desirable contact material to be used with a metallic silicide is polysilicon because it is inexpensive and easily formed. A typical process sequence forms the silicide, deposits and patterns a dielectric layer to expose selected portions of the underlying silicide, and deposits polysilicon at a temperature greater than 600.degree. C. See the paper, IEDM, 1991, "A Large Cell-Ratio and Low Node Leak 16M-bit SRAM Cell," by K. Yuzuriha et al., pp. 485-488, for a description of a polysilicon-silicide contact. This process sequence may also include ion implantation of a dopant, such as arsenic at an energy of approximately 100 Kev, to increase conductivity, and a rapid thermal anneal, typically approximately 20 seconds, at a temperature of approximately 900.degree. C. to activate the dopants. Patterning of the polysilicon and deposition of another dielectric frequently follows. The deposition temperature for this dielectric is frequently above 600.degree. C. to obtain adequate throughput.
However, when the material in the opening in the dielectric is examined carefully, it is found that there are spikes. The composition of the spikes is not known with certainty; however, it is believed that their primary constituent is silicon. Regardless of their precise composition, they adversely affect the quality of electrical contacts. For example, they may reduce the space available for the contact areas thereby increasing the contact resistance, and, if they are too high, penetrate a subsequently deposited dielectric and cause electrical shorts. Their elimination would therefore be desirable.